The present invention relates to cobalt-coated acicular magnetic iron oxide particles, and more particularly, to cobalt-coated acicular magnetic iron oxide particles in which a change of coercive force with temperature is small (the temperature dependence of the coercive force is small), which have a small switching field distribution and, hence, which are suitable as magnetic particles for magnetic recording media.
With a development of miniaturized and lightweight magnetic recording and reproducing apparatuses, magnetic recording media such as a magnetic tape and magnetic disk have been increasingly required to have a higher recording density and higher output characteristics.
In order to increase the magnetic recording density, it is necessary that iron oxide particles used are as fine as possible and the iron oxide particles have a high coercive force. This fact is described in, for example, on pages 185 to 187 of the COLLECTED DATA ON MAGNETIC RECORDING MEDIA (1985), published by Synthetic Electronics Research, " . . . The recent technical" innovation is remarkable, . . . higher-density recording has been developed. Therefore, much importance of improving the compactness, light weight and good operability on the basis of the high-density/short-wavelength recording technique in any of audio apparatuses, video apparatuses and floppy disk drives. As the magnetic film coating technique which meets such demand, the ultra-thin coating technique of a surface-smooth film for coating a film to a thickness of 1 to 2 .mu.m by using fine magnetic particles having a high coercive force, . . . as to a video tape, a high-grade (HG)-type tape using fine magnetic particles were ... in 1982, and as a tape, . . . by using ultrafine magnetic particles, . . . a remarkable improvement in the picture quality will be expected in the near future . . . "Even in the present state, there is no end to a demand for finer magnetic iron oxide particles.
Magnetic recording media are used under various thermal conditions. Especially, data storage tapes for computers are stored in the circumstances in which the temperature changes in a wide range. Such magnetic recording media are required not to lose the data due to the lowering of the coercive force even in a temperature range of 50 to 60.degree. C. There is, therefore, a strong demand for magnetic recording media in which a change of coercive force with temperature is small in a wide temperature range. For this purpose, the magnetic iron oxide particles themselves as a material are required to have a small change of the coercive force with temperature in a wide range.
In order to increase the output of a magnetic recording medium, it is necessary that the S.F.D. (Switching Field Distribution) of the magnetic recording medium is small. This is described in Japanese Patent Application Laid-Open (KOKAI) No. 63-26821 (1988): "FIG. 1 shows the relationship between the S.F.D., and the recording and reproducing output of the above-described magnetic disk . . . . The relationship between the S.F.D., and the recording and reproducing output is linear, as is obvious from FIG. 1, which proves that by using ferromagnetic particles having a small S.F.D. value, the recording and reproducing output enhance. That is, in order to increase the recording and reproducing output, the S.F.D. is preferably as small as possible. In order to obtain a higher output more than the ordinary one, it is necessary that the S.F.D. is not more than 0.6." For this purpose, magnetic iron oxide particles are required to have a width of the distribution of the coercive force as small as possible.
Various attempts have been made at producing acicular magnetic iron oxide particles to contain or coat various compounds in order to improve the properties thereof. There are, for example, a method of obtaining acicular magnetite particles by adding a phosphate to an aqueous suspension of acicular ferric oxide hydroxide particles to obtain acicular ferric oxide hydroxide particles with the phosphate adsorbed thereto, filtering, drying, heating, dehydrating and reducing the thus-obtained particles by an ordinary method (Japanese Patent Publication Nos. 55-6577 (1980) and 58-54487 (1983), and Japanese Patent Application Laid-Open (KOKAI) No. 57-113202 (1982)), a method of coating acicular magnetite particles with cobalt which are obtained by producing Si-containing goethite particles by the growth reaction of goethite seed crystals in the presence of Si, and reducing the goethite particles by an ordinary method (Japanese Patent Application Laid-Open (KOKAI) No. 5-335126 (1983)), a method of dissolving a magnesium salt in the aqueous dispersion of Co-coated magnetic iron oxide particles and then adding an alkali hydroxide so as to adhere the thus-produced magnesium hydroxide to the surface of the magnetic iron oxide particles (Japanese Patent Publication No. 62-50889 (1987)), a method of re-pulping a cake obtained by filtering and washing Co-coated magnetic iron oxide particles into a slurry, and adding an aqueous solution a magnesium salt so as to coat the surfaces of the particles with a hydroxide of magnesium ((Japanese Patent Publication No. 2-30563 (1990)), and a method of adding a magnesium salt to the washing water for washing the alkaline slurry of maghemite particle denatured by cobalt (Japanese Patent Application Laid-Open (KOKAI) No. 1-184801 (1989)).
More specifically, the method described in Japanese Patent Publication No. 55-6577 (1980) is a method of producing magnetic iron oxide particles as magnetic recording materials, comprising the steps of: producing acicular ferric oxide hydroxide particles by a wet reaction between a solution of a ferrous salt and an alkali; filtering the acicular ferric oxide hydroxide particles from the mother liquor and washing with water; suspending the particles in water; adding 0.1 to 2 wt % (calculated as PO.sub.3) of a phosphate to the ferric oxide hydroxide particles when the pH of the suspension is not less than 6 so as to disperse the acicular ferric oxide hydroxide particles; adjusting the pH of the suspension to 3 to 5; filtering out and drying the acicular ferric oxide hydroxide particles; and dehydrating under heating and reducing the acicular ferric oxide hydroxide particles by an ordinary method so as to produce acicular magnetite particles.
The method described in Japanese Patent Publication No. 58-54487 (1983) is a method of producing acicular magnetic iron oxide particles as magnetic recording materials, comprising the steps of: producing acicular ferric oxide hydroxide particles by a wet reaction between a solution of a ferrous salt and an alkali; adding 0.1 to 15 wt % (calculated as PO.sub.3 based on ferric oxide hydroxide particles) of a phosphate to the mother liquor after the end of the reaction; stirring the suspension; filtering out the acicular ferric oxide hydroxide particles; washing with water and drying; and dehydrating under heating and reducing the acicular ferric oxide hydroxide particles by an ordinary method so as to produce acicular magnetite particles.
The method described in Japanese Patent Application Laid-Open (KOKAI) No. 57-113202 (1982) is a method of producing acicular iron oxyhydrate fine particles, comprising the step of coating acicular magnetic iron oxide particles with a metal phosphate compound in the ratio of 0.05/100 to 5/100 as P/Fe.
The method described in Japanese Patent Application Laid-Open (KOKAI) No. 5-335126 (1993) is a method of producing acicular magnetic iron oxide particles, comprising the steps of passing oxygen-containing gas in a suspension of not less than pH 11 containing ferrous hydroxide which is obtained by reacting an aqueous ferrous salt solution with an aqueous alkali on conditions that spindle-shaped goethite particles are present as seed crystals either in the aqueous ferrous salt solution or in the suspension containing the ferrous hydroxide and before passing the gas, and that 0.2 to 5.0 atm % of an Si compound calculated as Si/Fe based on the total iron is added so as to produce acicular goethite particles by growing the seed crystals; and reducing the acicular goethite particles or the acicular hematite particles obtained by heating the acicular goethite particles at 300 to 700.degree. C., under heating in a reducing gas so as to obtain acicular magnetite particles; or further oxidizing the acicular magnetite particles so as to obtain maghemite particles.
The method described in Japanese Patent Publication No. 62-50889 (1987) is a method of producing a magnetic recording medium comprising the steps of: mixing at least one ferromagnetic particles selected from the group consisting of tri-iron tetroxide, .gamma.-ferrite, cobalt-treated .gamma.-ferrite, iron, iron-cobalt alloy, cobalt-chrome alloy and chrome oxide, a binder resin, an oxide and/or a hydroxide of at least one selected from the group consisting of magnesium, calcium, strontium, barium, zinc, cadmium, aluminum, gallium, yttrium and lead, and water, if necessary; and molding the mixture or applying the mixture to a nonmagnetic substrate.
The method described in Japanese Patent Application Laid-Open (KOKAI) No. 1-184801 (1989) is a method of producing magnetic iron oxide particles for magnetic recording, comprising the steps of: adding at least one selected from the group consisting of magnesium salt, calcium salt, strontium salt and barium salt to at least one selected from the group consisting of an alkaline slurry containing cobalt-modified .gamma.-iron oxide which is obtained by modifying .gamma.-iron oxide particles with cobalt in an aqueous alkaline solution, and water; and washing the alkaline slurry with the water.
Although cobalt-coated acicular magnetic iron oxide particles which are capable of suppressing a change of coercive force with temperature as much as possible and which have a small switching field distribution (S.F.D.), are now in the strongest demand, the known magnetic iron oxide particles described above cannot be said to sufficiently meet such demand.
More specifically, in any of the methods of producing magnetic iron oxide particles described in Japanese Patent Publication Nos. 55-6577 (1980) and 58-54487 (1983), and Japanese Patent Application Laid-Open (KOKAI) No. 57-113202 (1982), there is no process after the cobalt coating step. Therefore, the particles are not coated with a hydroxide of Mg, and as a result, the effect on the improvement of the switching field distribution (S.F.D.) and the improvement of a change of coercive force with temperature is not sufficient.
Similarly, in the method of producing magnetic iron oxide particles described in Japanese Patent Application Laid-Open (KOKAI) No. 5-335126 (1993), since the particles are not coated with a hydroxide of Mg after the cobalt coating, the effect on the improvement of a change of coercive force with temperature is not sufficient.
In the method of producing magnetic iron oxide particles described in Japanese Patent Publication Nos. 62-50889 (1987) and 2-30563 (1990) and Japanese Patent Application Laid-Open (KOKAI) No. 1-184801 (1989), since seed crystals containing P are not used at the time of coating the particles with cobalt as seed crystal particles, the effect on the improvement of the switching field distribution (S.F.D.) and the improvement of a change of coercive force with temperature is not sufficient.
Accordingly, the technical subject of the present invention is to solve the above-described problems and to produce cobalt-coated acicular magnetic iron oxide particles in which a change of coercive force with temperature is as small as possible and the switching field distribution (S.F.D.) is as smaller as possible, and therefore which are suitable for magnetic recording media.
As a result of studies undertaken by the present inventors, it has been found that in cobalt-coated acicular magnetic iron oxide particles produced by preparing acicular magnetite particles or acicular maghemite particles containing 0.05 to 1.00 wt % of phosphorus (calculated as P) and, if necessary, 0.05 to 1.00 wt % of an oxide of silicon (calculated as Si) as seed crystal particles; forming spinel ferrite layers containing cobalt on the surfaces of the seed crystal particles by heat-aging a mixed solution of an aqueous suspension containing the above-mentioned seed crystal particles, an aqueous alkali solution and a cobalt salt, or a mixed solution of an aqueous suspension containing the above-mentioned seed crystal particles, an aqueous alkali solution, a cobalt salt and a ferrous salt; adding a magnesium salt in the aqueous suspension containing the particles obtained; and adjusting the pH of the aqueous suspension to 7.5 to 10.5 so as to form magnesium hydroxide layers on the surfaces of the particles, a change of coercive force with temperature is small (the temperature dependence of the coercive fore is small) and the switching field distribution (S.F.D.) is small, so that the cobalt-coated acicular magnetic iron oxide particles are suitable as magnetic particles for magnetic recording media. The present invention has been achieved on the basis of this finding.